I sold my basket case CT26 as a core to a memeber here for $50 shipped. Not sure what a "needs rebuild" T4 would run for. It depends what size the turbine housing & style, and compressor housing size is. I've seen people sell .69 + AR tangential P-trim Vband turbine housings for $150 shipped on other forums. On center turbine housing could be had for around $100 shipped. That just the turbine housing. There not much of a market for compressor housings since most may have to be machined for the compressor wheel. Most people look to change the turbine housings to chance the characteristics of the turbo. I would think you could ask for roughly $200-250 shipped for a T4 that needs to be rebuilt. Maybe a little more, but it all depends on what size, style of housings, and manufacture of the turbo. You also need to be on the right forum to move something like that. Do you have to turbo yet? Please include specs & pics when you get it. If you could take off the housings and size up the compressor and turbine wheels that would be helpfull too when trying to sell.
Of course, you can get more if it was a good verifiable working turbo.